Project Summary: Elastic energy storage and return in tendons can save significant metabolic energy, but likely requires muscle force patterns with precise timing and magnitude. Neuromuscular and neurological impairments (e.g. stroke, spinal cord injury, cerebral palsy) and powered lower-limb robotic assistive devices can lead to altered patterns of muscle force output during locomotion. The broad goal of this research is to examine how tendon elasticity effects the mechanics of the contractile element of the musle-tendon unit under conditions of reduced muscle force output. The project will use an isolated muscle-tendon preparation and modeling, to test several hypotheses about the role of tendons in modulating muscle work during cyclic contraction. The specific aims are to (1) show that contractile element work does not decrease in proportion to muscle force during stretch-shorten contractions and (2) determine the effect of relative tendon compliance (i.e. the ratio of tendon length to muscle-tendon length) on the force-work relationship of the contractile element of muscle-tendon. The in vitro experiments will use a novel combination of sonomicrometry and muscle ergometry to separate the mechanical behavior of the muscle from the tendon during controlled stretch-shortening of bullfrog (Rana catesbeiana L.) plantaris longus muscle-Achilles tendon. To complement the in vitro experiments I will build an equivalent computer simulation of the muscle- tendon unit. Non-linear equations will characterize the force-length, force-velocity and force-activation properties of the contractile element and a linear spring will represent the series-elastic element. The rationale behind combining empirical and theoretical approaches is that (1) the muscle-tendon model can make predictions to be tested with carefully designed in vitro experiments and (2) the in vitro experiments on muscle-tendon can help test the validity of model simplifications. The proposed research will develop skills in both muscle physiology and non-linear computational modeling that will be invaluable to my future research program to develop bio-inspired lower limb assistive devices. Relevance: A fundamental understanding of integrated muscle-tendon function has important implications for solving problems relevant to human health. The proposed project will attract interdisciplinary interest from clinicians, physiologists, biomechanists, and engineers. The results will (1) aid in developing strategies for the rehabilitation of neurological and musculoskeletal disorders that limit muscle force production (2) contribute to improved design of prostheses and orthoses intended to assist the lower-limb during gait. [unreadable] [unreadable] [unreadable]